1. Field of the Invention
The present invention is directed in general to medical imaging systems, and more particularly to a Roentgen Stereophotogrammetric Analysis (RSA) calibration cage and method of operation thereof.
2. Description of the Related Art
Computerized X-ray techniques are well known for investigating motion of the skeletal system, such as implant migration, fracture stability, joint kinematics, etc. In the field of arthroplastic surgery, stereophotogrammetry has been used to evaluate the stability of implanted prostheses following surgery. Specifically, Roentgen Stereophotogrammetric Analysis (RSA) has been shown to be a particularly accurate radiographic method for the analysis of implant motion and wear (Selvik, G., 1989: “Roentgen Stereophotogrammetry, A method for the study of the kinematics of the skeletal system, Acta Orthop Scand Suppl 232, 1-51).
RSA is based on the acquisition of two simultaneous radiographs from two different X-ray foci, in order to obtain a three-dimensional reconstruction of a relationship between prosthesis implant and bone or between two or more skeletal segments at one or more orientations in a range or motion of the joint between skeletal segments. Recognizable reference points or markers (e.g. tantalum beads) are located in respective regions of interest (e.g. a prosthesis and a bone, or between a pair of bones). Different RSA configurations are known in the art, each utilizing a different calibration cage for different clinical applications: a uniplanar setup for large joints, such as the hip and spine wherein the patient is positioned above a control plate and a pair of co-planar fiducial plates; and a biplanar setup, for small joints, such as the knee, elbow and ankle wherein the patient is positioned between the fiducial plate and the control plate.
The RSA calibration cage includes two arrays of markers for creating a three-dimensional coordinate system. The stereo images are used to determine the position of the markers of the regions of interest in relation to the three-dimensional coordinate system established by the calibration cage. The projection of fiducial markers and control points attached to the cage are used to establish the exposure geometry (i.e. the relationship between the X-ray foci, calibration cage, and the x-ray detector). Successive reconstructions are created at predetermined intervals following surgery, and software is used to evaluate and track micromotion of the implant or object of interest with respect to a reference point.
The fiducial and control markers are typically distributed relatively evenly across the fiducial and control plates (conventionally referred to as fiducial and control planes) so as to overlap within the field of projection of the object points. During examination using a prior art uniplanar RSA cage, the patient is placed above the cage with two X-ray beams crossing at an angle of about 40 degrees with each other. This examination is specifically designed for large joints such as the hip and spine. For small joints such as knees, elbows, etc. a biplanar cage is used. The examination object is placed inside the cage and two X-ray beams cross each other at a perpendicular orientation.
The overall accuracy and precision of the RSA technique depends on the performance of each element of the procedure, from synthetic landmark insertion, stereo radiographic examination, radiographic measurement, to the final data analysis. Many studies have been performed to improve the outcome of these RSA elements. However, whereas most previous studies have addressed methodologies for optimizing specific steps in the RSA clinical process, few investigations have focused on the importance of RSA calibration cage design in stereo radiographic examination. Although the calibration cage is understood to be a crucial factor in determining the performance of stereo examination, the quantitative relationship between cage design and the accuracy and precision of RSA remains largely unaddressed.